1. Field of the Invention
The present invention relates to an internal combustion engine and a method for manufacturing the same. The present invention relates particularly to an internal combustion engine of which wall surface that faces a combustion chamber of an internal combustion engine is partially or entirely provided with an anodic oxidation coating and a method for manufacturing an internal combustion engine characterized by a method for forming the anodic oxidation coating.
2. Description of Related Art
An internal combustion engine such as a gasoline engine or a diesel engine is mainly configured of an engine block, a cylinder head, and pistons. The combustion chamber thereof is defined by a bore surface of a cylinder block, a piston top incorporated in the bore, a bottom surface of a cylinder head and tops of intake and exhaust valves disposed inside the cylinder head. As a recent internal combustion engine is demanded to be low fuel consumption, it is important to reduce the cooling loss. As one of countermeasures for reducing the cooling loss, a method of forming a heat-insulating coating of ceramic on an internal wall of a combustion chamber can be cited.
However, the above-mentioned ceramics generally has low thermal conductivity and high heat capacity. When an internal wall of a combustion chamber is made of ceramics, due to a steady increase of a surface temperature, an intake efficiency is deteriorated and knocking (irregular combustion due to confinement of heat inside a combustion chamber) is caused; accordingly, the ceramics is not prevailed at the present time as a coating material of an internal wall of a combustion chamber.
From this, a heat insulating coating formed on a wall surface of a combustion chamber is desirably formed of a material that has not only the heat resistance and heat insulating property but also low thermal conductivity and low heat capacity. That is, in order not to steadily raise a wall temperature, it is desirable that, in an intake stroke, the heat insulating coating is low in the heat capacity to decrease the wall temperature following an intake air temperature. Further, in addition to the low thermal conductivity and low heat capacity, a coating is desirably formed of a material that can withstand repeating stress of maximum combustion pressure and fuel injection pressure and thermal expansion and thermal shrinkage during combustion in a combustion chamber, and that is high in the adhesiveness with a base material such as a cylinder block.
A cylinder head in which on both of a bottom surface of a cylinder head and an interior surface of a water jacket defined in the cylinder head, a microporous silicon dioxide or aluminum oxide coating is formed by anodic oxidation is disclosed in Japanese Patent Application Publication No. 2003-113737 (JP 2003-113737 A). According to the cylinder head, since a microporous coating is disposed on both of a head bottom surface and an interior surface of jacket, a surface area of the head bottom surface and interior surface of jacket is expanded by the coating; accordingly, heat generated in the combustion chamber can be efficiently absorbed inside thereof via the coating. On the interior surface of jacket, heat absorbed inside can be efficiently released via the coating into cooling water. Accordingly, a cylinder head of which temperature increase is suppressed and the material is readily heated by absorbing heat or readily cooled by releasing heat can be obtained.
Like this, when an anodic oxidation coating is formed, on a wall surface that faces a combustion chamber of an internal combustion engine, an internal combustion engine that has low thermal conductivity and low heat capacity and is excellent in the heat insulating property can be formed. In addition to these performances, the anodic oxidation coating is further demanded to have excellent temperature swing characteristics. Here, the “temperature swing characteristics” is the characteristics where while having the heat insulating property, a temperature of the anodic oxidation coating follows a gas temperature inside a combustion chamber.
When the anodic oxidation coating is microscopically observed, there are many cracks on a surface thereof. Inside of the anodic oxidation coating, there are many defects that connect to the cracks. It is general that many voids that form these cracks and defects are present over from a surface of the coating to the inside thereof.
The present inventors have identified that these cracks and defects have a dimension in the range of about 1 to 10 μm.
Further, inside of the anodic oxidation coating, in addition to the voids of micro-order, also many fine holes of nano-order (nano-hole) are present.
An anodic oxidation coating generally includes voids such as micro-order surface cracks and internal defects and many nano-holes of nano-order. It has been identified according to the present inventors that while the micro-order voids are desirable to be sealed (embedded, clogged) from the viewpoint of the coating strength, many nano-holes are desirable to remain in the anodic oxidation coating in a state having pores of nano-size from the viewpoint of the temperature swing characteristics.
Here, as a conventional technology that seals the micro-order surface cracks (voids), a corrosion-resistant surface treatment article and a method for producing the same disclosed in Japanese Patent Application Publication No. 2005-298945 (JP 2005-298945 A) can be cited.
JP 2005-298945 A discloses a technology where a silicon component derived from perhydropolysilazane or a polycondensate thereof is filled in the surface cracks to seal.
As disclosed in JP 2005-298945 A, when relatively large size surface cracks are sealed by filling perhydropolysilazane, the voids are sealed and the coating strength can be improved. However, only by filling perhydropolysilazane in an anodic oxidation coating, the nano-holes present inside the coating are also sealed. Accordingly, it is difficult to form an anodic oxidation coating excellent in the temperature swing characteristics.
The present invention provides an internal combustion engine that is provided with an anodic oxidation coating that has low thermal conductivity and low heat capacity, is excellent in heat insulating property, and is excellent in the temperature swing characteristics on a part or an entirety of a wall surface that faces a combustion chamber, and a method for manufacturing the internal combustion engine.